Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley 17 .
Color-magnitude diagrams ( CMDs ) are constructed and reach well past the main-sequence turn-off for both clusters .
Cluster ages are determined by means of isochrone fitting to the cluster CMDs .
These fits are constrained to agree with spectroscopic metallicity and reddening estimates .
Cluster ages are determined to be 7.0 \pm 0.5 Gyr for NGC 188 , and 10.0 \pm 1.0 Gyr for Berkeley 17 , where the errors refer to uncertainties in the relative age determinations .
These ages are compared to the ages of relatively metal-rich inner halo/thick disk globular clusters and other old open clusters .
Berkeley 17 and NGC 6791 are the oldest open clusters with an age of 10 Gyr .
They are 2 Gyr younger than the thick disk globular clusters .
These results confirm the status of Berkeley 17 as one of the oldest known open cluster in the Milky Way , and its age provides a lower limit to the age of the Galactic disk .